This invention relates to a process for purifying esters such as methyl acetate and, more particularly, the invention relates to a process for removing carbonyl impurities from such esters.
Methyl acetate is used as a feedstock in carbonylation processes such as to produce acetic anhydride or with methanol to coproduce acetic acid and acetic anhydride. It is also known to produce acetic acid only when methyl acetate is fed as a supplement or replacement feedstock for methanol in processes in which a stoichiometric amount of water is cofed with the methyl acetate. Unfortunately, methyl acetate made under certain esterification processes contains carbonyl impurities. For example, large volumes of impure methyl acetate are produced as a byproduct during the manufacture of polyvinyl alcohol from polyvinyl acetate. This impure methyl acetate has been recovered by a very expensive process in which the methyl acetate is converted to acetic acid by hydrolysis. In a typical process for producing polyvinyl alcohol, polyvinylacetate is reacted with methanol in the presence of a base to yield polyvinylalcohol and methyl acetate. This methyl acetate stream is contaminated with a variety of carbonyl components including acetaldehyde. Prior methods of recovering this methyl acetate stream have involved contacting the methyl acetate stream with an acid catalyst such as an acid resin in the presence of water which converts the methyl acetate to acetic acid and methanol which can be reused to convert the polyvinylacetate to polyvinylalcohol.
The presence of carbonyl impurities is very objectionable in many of the uses to which methyl acetate is put including carbonylation processes for the production of acetic acid or acetic anhydride or coproduction of these two materials. During the production of acetic acid by the carbonylation of methanol or methyl acetate and water, it has now been discovered by the present inventors that carbonyl impurities such as acetaldehyde, acetone, methyl ethyl ketone, crotonaldehyde, etc. react to form aldol condensation products and/or react with iodide catalyst promoters to form multicarbon alkyl iodides. If contained in the final acetic acid product, these impurities cause quality problems. In the production of acetic anhydride or in acetic acid/acetic anhydride coproduction by carbonylation of methyl acetate, it is known that undesirable high boiling tars are formed in the catalyst solution. The tars are believed to be formed by aldol condensation of the aldehydes and ketones as well as by reaction of the carbonyl and aldol products with the formed acetic anhydride. The tars bind with or otherwise entrap the Group VIII metal carbonylation catalyst and, upon precipitation from the catalyst solution, remove the catalyst. Thus, not only has the precipitated tar become an environmental problem, the operation of the commercial carbonylation process has been degraded and made more costly as make-up catalyst is required. In order to solve this problem, the prior art has treated the carbonylation catalyst recycle stream to remove undesirable components such as acetone or attempted to remove the metal carbonylation catalyst from high boiling residues. The treatment of the symptom is costly and not overly effective.
Unfortunately, it is difficult to remove the minor amounts of carbonyl impurities present in methyl acetate by conventional means such as distillation inasmuch as such impurities have boiling points close to that of methyl acetate.
Various processes have been suggested to remove the minute amounts of carbonyl impurities from esters. For example, it is known to remove aldehyde impurities from esters by contacting the ester with an amino compound, which amino compound reacts with the aldehydes. The reaction products are subsequently separated from the ester. An example of such a process is disclosed in U.S. Pat. No. 1,963,968 in which an ester such as methyl acetate is contacted with an amino compound such as aniline, phenylhydrazine, diphenylamine, hydroxylamine or ammonia, etc. The process involves vaporizing the ester and passing the amino compound in a liquid state or vaporizing the amino compound and passing same through the ester in a liquid state or contacting both the ester and amino compound in the liquid state in a reaction vessel and refluxing the liquids until the amino compound reacts with the aldehydes contained in the ester. The ester is recovered in a purified state by fractional distillation.
U.S. Pat. No. 3,290,363 discloses removing aldehydes from vinyl acetate by treating the vinyl acetate with an aromatic amine such as aniline and naphthylamines.
A similar process for the purification of acrylonitrile is disclosed in U.S. Pat. No. 2,770,644. In this process, acrylonitrile which contains minor amounts of methyl vinyl ketone is contacted with an aqueous solution of hydroxylamine hydrochloride, the mixture allowed to separate into two layers and the acrylonitrile separated from the reaction mixture. A pure acrylonitrile can be obtained by distillation.
While the removal of minute quantities of carbonyls from organic streams by a process which involves contacting the organic stream with amino compounds and subsequent separation by distillation has been successful in removing the carbonyl impurities, the process has several disadvantages. For one, typically large excesses of the amino compound relative to the amount of carbonyls present in the organic stream are utilized. Thus, material costs as well as energy costs involved in the separation stage such as by distillation of the organic stream from the reacted impurities are disadvantageous.
It has also been discovered by the present inventors that the product which is formed by the reaction of an aldehyde impurity and an amino compound can be converted to a nitrile during the distillation stage used to separate the reactants from the purified organic feed. Thus, acetaldehyde has been found to convert to acetonitrile by the prior art process. The formed nitriles do not easily separate along with the other impure reaction products and remain in the ester feed stream. The presence of nitriles such as acetonitrile in an ester stream is a great disadvantage in many processes including the carbonylation of methyl acetate to acetic anhydride or to the corresponding coproduction of acetic acid and acetic anhydride as described above. Also keto impurities may form undesirable nitrogenous derivatives upon reaction with the amino compound and subsequent heating during distillation. Such nitrogenous keto species may also be difficult to separate from like boiling product esters.
The prior art has not recognized the problem of nitrile formation during processes of removing carbonyls from ester streams by amino compound addition. Accordingly, there is a need to purify organic streams such as methyl acetate containing minor amounts of aldehydes without producing nitriles.